This invention relates generally to an Arc Fault Detection device, and more particularly to an arc fault detection device comprising a two stage arc sensor. The first stage is a current transformer designed for high sensitivity but which may saturate during high level arc fault currents causing the loss of the secondary arc sense signal. The second arc fault sensor stage senses the voltage dropped across the impedance of the primary of the current transformer, or the voltage dropped across the resistance of a section of the load carrying bus which forms the power connection through the detector device, or the voltage drop across both.
A percentage of fires each year are caused by electrical branch circuit line arcing which is of a duration, and at a level, which does not activate the thermal or magnetic trip elements in conventional circuit breakers in time to prevent a fire.
Arc detection is an enhancement to thermal magnetic overload detection typically used in circuit breakers, which alone may not detect and respond to arc faults. A number of devices for detecting arc faults and methods of detection have been used in the past. These include the use of E and B field arc sensors, detecting the amplitude of the rate of change of current signals when an arc fault occurs, the use of non-overlapping band pass filters to detect white noise characteristic of arcs, and detecting the disappearance of signals indicating the presence of arcs near zero current crossings. While some of these techniques are more or less effective, they require relatively sophisticated arc sensors and circuits. Heretofore, most arc detection circuits have been incorporated in circuit breakers.
There is a need for simple economical arc fault detectors that can be included in wiring devices such as receptacles, plugs or in-line devices, and that offer the same protection as an arc fault detector incorporated in a circuit breaker, but at lower cost. There is a need for an arc fault circuit detector in wiring devices that can be provided at a reduced cost compared with arc fault circuit detecting circuit breakers that is comparable to the reduction in cost between ground fault interrupting receptacles and ground fault interrupting circuit breakers.
This invention relates to an arc fault detector with a two stage arc sensor which permits a much smaller and less expensive current transformer sensor without sacrificing the detectors ability to respond to a broad range of arc fault currents. This allows a less expensive and smaller overall circuit which can be constructed to fit into a wiring sized device and which may also permit a dual purpose arc and ground fault detection circuit.
It is an object of this invention to provide an arc fault circuit interrupter that employs an electrical circuit that is simple enough, inexpensive enough and small enough to be included in wiring devices. It is another object of this invention to provide an arc fault circuit interrupter that is sensitive to relatively low amplitude series arc faults of at least 5 amps of arc current, typically in series with the load and commonly referred to as Type A arc faults.
It is another object of this invention to provide an arc fault circuit interrupter that detects parallel or line to line arcs, producing currents of 75 amps or more, commonly referred to as Type B arc faults.
Briefly stated, and in accordance with a presently preferred embodiment of the invention, an arc fault detector for detecting electric power lines includes a two stage arc sensor. The first stage is a current transformer designed for high sensitivity to arc faults but which may saturate and lose detection capability during arc currents at 75A and higher. The second arc fault sensor stage senses the voltage across the impedance of the primary of the current transformer, or senses the voltage across the resistance of a section of the load carrying bus which forms the connection through the detector device, or of both, during an arc fault, which does not saturate.